Method and apparatus for pre-distortion of an X-DSL line driver

ABSTRACT

An apparatus and method is provided for minimizing in channel distortion in an X-DSL line driver is disclosed. The apparatus may be incorporated in an existing X-DSL architecture without additional circuitry. Out of band monitoring of a channel is implemented to adaptively minimize out of band interference and in band distortion. A unique training sequence, suitable for DMT line codes is set forth. The training sequence allows a full spectral characterization of the downstream signal space with a single upstream monitoring tone. The invention may be used with multi-channel X-DSL line drivers interfacing with any of a number of multi-channel supply architectures. The apparatus may be applied with equal advantage to communication protocols other than X-DSL. The apparatus may be applied with equal advantage in wired and wireless media.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of copending U.S. application Ser.No. 09/740,620 filed Dec. 18, 2000 entitled “Method and Apparatus forPredistortion of an X-DSL Line Driver” which claims the benefit of priorfiled Provisional Applications No. 60/172,404 filed on Dec. 17, 1999entitled “Pre-Distortion System for ADSL and G.Lite Transmission”. Eachof the above-cited applications is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The field of the present invention relates in general to an adaptivepre-distortion systems and more particularly to a method and apparatusfor adaptive pre-distortion in particular but not exclusively of X-DSLline drivers.

2. Description of the Related Art

North American Integrated Service Digital Network (ISDN) Standard,defined by the American National Standard Institute (ANSI), regulatesthe protocol of information transmissions over telephone lines. Inparticular, the ISDN standard regulates the rate at which informationcan be transmitted and in what format. ISDN allows full duplex digitaltransmission of two 64 kilo bit per second data channels. These datarates may easily be achieved over the trunk lines, which connect thetelephone companies' central offices. The problem lies in passing thesesignals across the subscriber line between the central office and thebusiness or residential user. These lines were originally constructed tohandle voice traffic in the narrow band between 300 Hz to 3000 Hz atbandwidths equivalent to several kilo baud.

Digital Subscriber Lines (DSL) technology and improvements thereonincluding: G.Lite, ADSL, VDSL, HDSL all of which are broadly identifiedas X-DSL have been developed to increase the effective bandwidth ofexisting subscriber line connections, without requiring the installationof new fiber optic cable. An X-DSL modem operates at frequencies higherthan the voice band frequencies, thus an X-DSL modem any operatesimultaneously with a voice band modem or a telephone conversation.Currently there are over ten discrete X-DSL standards, including:G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc.

One of the factors limiting the bandwidth or channel capacity of any ofthe above discussed X-DSL protocols is interchannel interference.Electronic amplifiers and line drivers employed in many communicationsystems inherently distort signals as they amplify them. Furthermore, tomaximize power efficiency these line drivers are often operated near thesaturation point where the input/output power characteristics becomenonlinear. Amplitude modulation causes distortion to become dependent onthe input signal with a result of the amplified output signal is nolonger simply an amplified replica of the input signal. Unfortunately iflinear modulation with a fluctuating envelope is used in conjunctionwith nonlinear amplification, spectral spreading into adjacent cans willoccur thereby interfering with communications.

What is needed are approaches to reducing in band distortion and out-ofband interference for X-DSL line drivers.

SUMMARY OF THE INVENTION

An apparatus and method is disclosed for minimizing in channeldistortion in an X-DSL line driver. The apparatus may be incorporated inan existing X-DSL architecture without additional circuitry. Out of bandmonitoring of a channel is implemented to adaptively minimize out ofband interference and in band distortion. A unique training sequence,suitable for DMT line codes is set forth. The training sequence allows afull spectral characterization of the downstream signal space with asingle upstream monitoring tone.

During a setup phase one or more device parameters are varied for eachof a plurality of tones to characterize the out of band leakage of thetransmit line. At the close of the setup phase device parameters areselected which result in the lowest overall out-of-band interference onthe upstream signal line.

Next, during a configuration phase the same training sequence isimplemented at varying amplitudes across the downstream signal space.Using a leakage model of the out-of-band interference and an initialdevice model the expected out-of-band interference is calculated foreach tone in the training sequence. The error between the calculatedvalue of the out-of-band interference and the received monitor tone isdetermined and the initial device model is updated to reflect actualoperating conditions.

At the completion of the configuration phase an inverse channel model isgenerated using the values accumulated in the updated device modeltable. This inverse model is used to distort each transmitted symbolsequence to linearize the line driver and other portions of the transmitcircuit during actual operation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

FIG. 1 shows a communication system with a pair of multi-modemulti-channel modem line cards coupled to one another by a binder ofsubscriber lines between a public switched telephone network (PSTN)central office (CO) and a remote site.

FIG. 2 is a detailed hardware block diagram of one of the modem linecards shown in FIG. 1.

FIGS. 3A-C are detailed logical block diagrams showing basic logicblocks associated with: the setup, configuration and operational phasesrespectively.

FIG. 4A is a graph of an ADSL signal space showing the asymmetricalupstream and downstream portions thereof.

FIG. 4B is a table showing the selected DMT tones for the downstreamtraining sequence which result in a single monitor tone in the upstreamchannel.

FIG. 5 is a process flow diagram of the setup, configuration andoperational phases.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An apparatus and method is provided for minimizing in channel distortionin an X-DSL line driver is disclosed. The apparatus may be incorporatedin an existing X-DSL architecture without additional circuitry. Out ofband monitoring of a channel is implemented to adaptively minimize outof band interference and in band distortion. A unique training sequence,suitable for DMT line codes is set forth. The training sequence allows afull spectral characterization of the downstream signal space with asingle upstream monitoring tone. The invention may be used withmulti-channel X-DSL line drivers interfacing with any of a number ofmulti-channel supply architectures. The apparatus may be applied withequal advantage to communication protocols other than X-DSL. Theapparatus may be applied with equal advantage in wired and wirelessmedia.

FIG. 1 shows a communication system with a pair of multi-modemulti-channel modem line cards coupled to one another by a binder ofsubscriber lines between a public switched telephone network (PSTN)central office (CO) and a remote site. The system includes a CO 100 anda remote line card 156 positioned at a remote terminal 150. The CO andremote line card are coupled to one another via a subscriber line binder170 which includes individual subscriber lines 172,174, 176.

Each of the subscriber line connections terminates on the CO end, in theframe room 102 of the CO. From this room connections are made for eachsubscriber line via splitters and hybrids to both a DSLAM 104 and to thevoice band racks 106. The splitter shunts voice band communications todedicated line cards, e.g. line card 112 or to a voice band modem pool(not shown). The splitter shunts higher frequency X-DSL communicationson the subscriber line to a selected fine card, e.g. line card 116,within DSLAM 104. The line cards of the current invention are universal,meaning they can handle any current or evolving standard of X-DSL andmay be upgraded on the fly to handle new standards.

Voice band call set up is controlled by a Telco switch matrix 114 suchas SS7. This makes point-to-point connections to other subscribers forvoice band communications across the public switched telephone network132. The X-DSL communications may be processed by a universal line cardsuch as line card 116. That line card includes a plurality of AFE's118-120 each capable of supporting a plurality of subscriber Lines. TheAFEs are coupled via a packet based bus 122 to the DSP 124. Fordownstream communications from the CO to the remote site, the DSPmodulates the data for each communication channel, the AFE transformsthe digital symbol packets assembled by the DSP and converts them to ananalog signal which is output on the subscriber line associated with therespective channel. For upstream communications from the remote site tothe CO each received channel is converted within the corresponding AFEto a digitized data sample which is sent to the DSP for demodulation.The DSP is capable of multi-protocol support for all subscriber lines towhich the AFE's are coupled. Communications between AFE's and DSP(s) maybe packet based, in which embodiment of the invention a distributedarchitecture such as will be set forth in the following FIG. 2 may beimplemented. The line card 116 is coupled to a back-plane bus 128 whichmay be capable of offloading and transporting low latency X-DSL trafficbetween other DSPs for load balancing. The back-plane bus of the DSLAMalso couples each line card to the Internet 130 via server 108. Each ofthe DSLAM line cards operates under the control of a DSLAM controller110 which handles global provisioning, e.g. allocation of subscriberlines to AFE and DSP resources. The various components on the line cardform a plurality of logical modems each handling upstream and downstreamcommunications across corresponding subscriber lines. In an alternateembodiment of the invention discrete modems would each couple to anassociated one of the subscriber lines rather than the logical modemshown. When an X-DSL communication is established on a subscriber line,a specific channel identifier is allocated to that communication. Thatidentifier is used in the above mentioned packet based embodiment totrack each packet as it moves in an upstream or downstream directionbetween the AFE and DSP.

At the remote site a similar line card architecture is shown for liecard 156 which forms a plurality of logical modems connected tocorresponding ones of subscriber lines 172, 174, 176. That line cardincludes AFEs 158, a packet bus 160 and a DSP. In an alternateembodiment of the invention the termination at the remote site 150 wouldbe a set of discrete modems each coupled to an associated one of thesubscriber lines rather than the logical modem shown. These modules, AFEand DSP, may be found on a single universal line card, such as line card116 in FIG. 2. They may alternately be displaced from one another onseparate line cards linked by a DSP bus. In still another embodimentthey may be found displaced from one another across an ATM network.There may be multiple DSP chipsets on a line card. In an embodiment ofthe invention the DSP and AFE chipsets may include structures set forthin the figure for handling of multiple line codes and multiple channels.

FIG. 2 is a detailed hardware block are hardware block diagram of one ofthe modem line cards shown in FIG. 1. FIG. 2 shows a packet basedmulti-channel transmission architecture within which the currentinvention may be implemented. In this architecture a DSP 124 handlesprocessing for a number of channels of upstream and downstreamsubscriber line communications via a number of AFE's. Each AFE in turnaccepts packets associated with a plurality of subscriber lines to whicheach AFE is coupled. FIG. 2 shows a packet based raw data processingboth between a DSP and AFE as well as within each DSP and AFE. Packetprocessing between DSP and AFE modules involves transfer of bus packets294 each with a header portion 296 and data portion 298. The headercontains information correlating the data with a specific channel anddirection, e.g. upstream or downstream, of communication. The dataportion contains for upstream traffic digitized samples of the receiveddata for each channel and for downstream packets digitized symbols forthe data to be transmitted on each channel. Packet processing within aDSP may involve device packets 286. The device packets may include aheader 288, a control portion 290 and a data portion 292. The headerserves to identify the specific channel and direction. The header mycontain control information for the channel to be processed. The controlportion 290 may so contain control portions for each specific componentalong the transmit or receive path to coordinate the processing of thepackets. Within the AFE the digitized data generated for the received(upstream data) will be packetized and transmitted to the DSP. Fordownstream data, the AFE will receive in each packet from the DSP thedigitized symbols for each channel which will be modulated in the AFEand transmitted over the corresponding subscriber line.

These modules, AFE and DSP, may be found on a single universal linecard, such as line card 116 in FIG. 1. They may alternately be displacedfrom one another on separate lie cards linked by a DSP bus. In stillanother embodiment they may be found displaced across an ATM network.

DSP line card 116 includes one or more DSP's. In an embodiment of theinvention each may include structures set forth in the figure forhandling of multiple line codes and multiple channels. The line cardincludes, a DSP medium access control (MAC) 200 which handles packettransfers to and from the DSP bus 122. The MAC couples with a packetassembler/disassembler (PAD) 202. For received DSP bus packets, the PADhandles removal of the DSP bus packet header 296 and insertion of thedevice header 288 and control header 290 which is part of the devicepacket 286. The content of these headers is generated by the coreprocessor 212 using statistics gathered by the de-framer 222. Thesestatistics may include gain tables, or embedded operations channelcommunications from the subscriber side. The PAD embeds the requiredcommands generated by the core processor in the header or controlportions of the device packet header. Upstream device packets (Receivepackets) pass into a first-in-first-out FIFO buffer 208 which iscontrolled by FIFO controller 206. These packets correspond withmultiple protocols and multiple channels. Each is labeled accordingly.The receive processing engine 204 in this case a DMT engine fetchespackets and processes the data in them in a manner appropriate for theprotocol, channel and command instructions, if any, indicated by theheader. The processed data is then passed to the De-Framer and ReedSolomon Decoder 222. This module reads the next device packet andprocesses the data in it in accordance with the instructions orparameters in its header. The processed de-framed data is passed to thefinal FIFO buffer 226 which is controlled by controller 204. That datais then passed to the ATM pad 228 for wrapping with an ATM header andremoval of the device header. The ATM MAC 230 then places the data withan ATM packet on the ATM network 130 (see FIG. 1).

Control of the receive modules, e.g. DMT engine 204 and de-framerdecoder 222 as well as sub modules thereof is implemented as follows.The core processor 210 has DMA access to the FIFO buffer 226 from whichit gathers statistical information on each channel including gaintables, or gain table change requests from the subscriber as well asinstructions in the embedded operations portion of the channel. Thosetables 214 are stored by the core processor in memory 212. When a changein gain table for a particular channel is called for the core processorsends instructions regarding the change in the header of the devicepacket for that channel via PAD 202 and writes the new gain table to amemory which can be accessed by the appropriate module, i.e. DMT module204 or the appropriate sub module thereof as a packet corresponding tothat channel is received by the module. This technique of in bandsignaling with packet headers allows independent scheduling of actionson a channel by channel basis in a manner which does not require thedirect control of the core processor. Instead each nodule in thetransmit path can execute independently of the other at the appropriatetime whatever actions are required of it as dictated by the informationin the device header which it reads and executes.

This device architecture allows the DSP transmit and receive paths to befabricated as independent modules or sub modules which respond to packetheader control information for processing of successive packets withdifferent X-DSL protocols, e.g. a packet with ADSL sample data followedby a packet with VDSL sampled data. Within the DMT Rx engine 204 forexample, there may be sub modules with independent processing capabilitysuch as: a time domain equalizer, a cyclic prefix remover, a DFT, a gainscalar, a trellis decoder and a tone reorderer, as well as filters, awindowers . . . etc. Each of these sub modules has its counterpart onthe DMT Tx engine 220 in the transmit path. Each of these mayindependently respond to successive device headers to change parametersbetween successive packets. For example as successive packets fromchannels implementing G.Lite, ADSL and VDSL pass through the DMT Txengine the number of tones will vary from 128 for G.lite, to 256 forADSL, to 2048 for VDSL. The framer and de-framer will use protocolspecific information associated with each of these channels to look fordifferent frame and super frame boundaries. The DMT receive engine 204implements processes for monitoring a monitor tone on the upstreamchannel during the setup and configuration phases of the method foradaptively minimizing out of band interference and in band distortion.The measured level of each tone is maintained by processor 210 in memory212. This same memory nay be utilized for calculating the inversechannel model for each of the channels to determine the amount ofpre-distortion to be applied to downstream data on each of the channels.

On the downstream side, i.e. Transmit, the same architecture applies.ATM data which is unwrapped by PAD 228 is re-wrapped with a deviceheader the contents of which are again dictated by the core processor210. That processor embeds control information related to each channelin the packets corresponding to that channel. The device packets arethen passed to the FIFO buffer 232 which is controlled by controller234. The Framer and RS encoder 236 and or sub modules thereof thenprocesses these packets according to the information contained in theirheader or control portions of each device packet. The Framer thenupdates the device packet header and writes the resultant device packetto the DMT transmit module 220. This module accepts the data andprocesses it for transmission. Transmission processing may include: toneordering, trellis encoding, gain scaling, an IDFT, and cyclic prefixmodules each with independent ability to read and respond to deviceheaders. During the training and setup phases the training module 238generates a unique set of tones across the downstream signal space whichresults in out-of-band interference on a single tone of the upstreamchannel. (See FIGS. 3-5). During the operational phase an inversechannel model maintained in memory 212 is utilized to pre-distort eachdownstream channel to linearize the in band signal and to minimize outof band interference with the upstream channel. Separate inverse channelmodels are maintained for each channel.

From the DMT Tx engine 220 each updated device packet with a digitizedsymbol(s) for a corresponding channel is placed in the FIFO buffer 216under the control of controller 218. From this buffer the device packetis sent to PAD 202 where the device header is removed. The DSP PADplaces the DSP packet 294 with an appropriate header onto the DSP bus122 for transmission to the appropriate AFE and the appropriate channeland subscriber line within the AFE.

Because the data flow in the AFE allows a more linear treatment of eachchannel of information an out of band control process is utilized withinthe AFE. In contrast to the DSP device packets which are used tocoordinate various independent modules within the DSP the AFEaccomplishes channel and protocol changeovers with a slightly differentcontrol method.

A packet on the bus 294 directed to AFE 122 is detected by AFE MAC 240on the basis of information contained in the packet header. The packetis passed to PAD 242 which removes the header 296 and sends it to thecore processor 244. The packet's header information including channel IDis stored in the core processor's memory 248. The information iscontained in a table 266. The raw data 298 is passed to a FIFO buffer252 under the control of controller 250. Each channel has a memorymapped location in that buffer.

On the transmit path, the interpolator 254 reads a fixed amount of datafrom each channel location in the FIFO buffer. The amount of data readvaries for each channel depending on the bandwidth of the channel. Theamount of data read during each bus interval is governed by entries inthe control table for each channel which is established during channelsetup and is stored in memory 248. The interpolator up samples the dataand low pass filters it to reduce the noise introduced by the DSP.Implementing interpolation in the AFE as opposed to the DSP has theadvantage of lowering the bandwidth requirements of the DSP bus 310A.From the interpolator data is passed to the digital-to-analog converter(DAC) 260. The DAC converts the digitized symbol for each of the inputsignals on each of the input signal lines/channels to correspondinganalog signals. These analog signals are introduced to the amplificationstage 262, from which they are coupled to corresponding subscriberlines. The amplification stage is coupled to a power supply 266. Each ofthe transmit modules 254, 260, 262 is coupled to the control processor244. During the setup phase the controller 258 varies one or more inputparameters, e.g. voltage, current, etc. of one or more of the transmitmodules as each tone is generated on the downstream transmission path ofthe DSP. The sequence is synchronized using the packet headersassociated with each channel and a control sequence which may bedownloaded from the DSP to the AFE prior to the setup phase for eachchannel.

The parameters for each of the modules 254, 260, 262, i.e. filtercoefficients, amplifier gain etc. are controlled by the core processorusing control parameters stored during session set up. For example,where successive packets carry packets with G.Lite, ADSL, and VDSLprotocols the sample rate of the filter parameters for filter 254 andthe gain of the analog amplifiers within stage 262 will vary for eachpacket. This “on the fly” configurability allows a single transmit orreceive pipeline to be used for multiple concurrent protocols.

On the upstream path, the receive path, individual subscriber linescouple to individual line amplifiers, e.g. 270-272, through splitter andhybrids (not shown). Each channel is passed to dedicated ADC modules274-276. Next each channel may be subject to further filtering anddecimation 278. As discussed above in connection with the transmit path,each of these components is configured on the fly for each new packetdepending on the protocol associated with it. Each channel of data isthen placed in a memory mapped location of FIFO memory 282 under thecontrol of controller 280. Scheduled amounts of this data are moved toPAD 242 during each bus interval. The PAD wraps the raw data in a DSPheader with channel ID and other information which allows the receivingDSP to properly process it.

In an alternate embodiment of the invention the sane packet basedcontrol principal may be used in both the transmit and receive path toimplement not only multiple protocols concurrently but alternate linescodes, e.g. CAP/QAM.

FIGS. 3A-C are detailed logical block diagrams showing basic logicblocks associated with: the setup, configuration and operational phasesrespectively of the current invention. These blocks may be implementedwithin the existing processing units of one or more physical or logicalmodems.

In FIG. 3A a training module 238 is shown coupled to the tone generator300 which is coupled to the transmit signal path 302. The trainingmodule causes the tone generator to generate the tones shown in thefollowing FIG. 4 across the downstream signal space. As each tone isgenerated over repeated symbol sets the DAC performs the analogconversion and the corresponding amplified signal is generated byamplifier 262 on downstream signal line 304 which couples the transmitpath to the hybrid 306. The hybrid is shown coupled to the transformer308 which in turn is coupled to the subscriber line 310. The currentinvention nay be applied with equal advantage in wireless media as well.During the generation of each tone the controller 258 effects a changein one or more of the device control parameters of the transmit pathmodules, e.g. the amplifier 262. The bias, or input voltage from powersupply 266 may be varied for example. Alternately parameters such astemperature may be varied. During the generation of each tone, and thevariation of the device control parameters a monitor tone is detected inthe out-of-band upstream channel 314. This tone is amplified in upstreamamplifier 272 and digitized in analog-to-digital converter (ADC) 276 andsupplied in digitized form to the tone detector 316. The tone detectormay be used to synchronize the controller 258. The tone detectorascertains relevant parameters for the monitor tone such as itsamplitude and supplies these to processor 318. This processor stores foreach tone and each parameter variation during each tone interval thedetected levels of the monitor tone. At the close of this phase ofoperation the processor 318 determines which among the parameters offersthe lowest overall out-of-band leakage across the downstream frequencyspace. Once this determination has been made the controller locks thesecontrol parameters for each of the measured transmit path components,i.e. the amplifier/line driver 262 during subsequent operation.

Next in FIG. 3B the configuration phase of operation is shown with theassociated logical and physical components for effecting this phase ofoperation. In this phase the same tone sequence is generated by the tonegenerator on the transmit path. The control parameters for the DAC 260and amplifier are locked, to minimize out-of-band interference. Theresultant monitor tone is amplified in amplifier 272 and digitized inADC 276 and provided as an input to error detector 326. The output ofthe tone generator is also supplied to processor 318. The processorworks with a leakage model table and device model table stored in memory322 to estimate the level of out-of-band signal for the correspondingtone. The estimated level is compare with the actual level in errordetector 326 and the difference is recorded as an offset within thecorresponding row of the device parameter table in memory 322. Thedevice parameter table may store relevant functional, mathematical ortabular models for the transmit path components such as the amplifier.For an amplifier the device model would store for example, voltage vs.current curves for the amplifier 262. The leakage model table wouldmodel out-of-band monitor tone levels for a variety of in-band-tones. Atthe conclusion of the configuration phase the device table and theoffsets recorded therein are used by processor 318 to generate aninverse channel model.

Next, in FIG. 3C the inverse non-linear channel model 350 stored inmemory 322 is used to pre-distort each digitized symbol to linearize theperformance of the transmit path. To reduce out of band signal emissionsto an acceptable minimum the amplifier input signal is conventionally“pre-distorted” before it is fed into the amplifier. Before the signalis amplified, an estimate is made of the manner in which the amplifierwill distort the particular input signal by amplifying that signal. Thesignal to be amplified is then “pre-distorted” by apply to it atransformation which is estimated to be complementary to the distortingtransformation which the amplifier itself will apply to the amplifiedsignal. In theory, the effective pre-distorting transformation isprecisely canceled out by the amplifiers distorting transformation, toyield an undistorted, amplified replica of the input signal. Suchamplifiers are said to be “linearized” in the sense that the outputsignal is proportional to the input signal, thereby eliminating thegeneration of out-of-band components and minimizing in band distortion.

FIG. 4A is a graph of an ADSL signal space showing the asymmetricalupstream and downstream portions thereof. The downstream frequency space402 is implemented at frequencies between 138 kHz and 1.1 Mhz. Theupstream frequency space 400 is implemented between frequencies 138 kHzand 25 kHz. A plurality of dual tone sets 406 and 412 are shown spacedapart within the downstream channel. The transmission of the first ofthese tone pairs 406 in the downstream frequency space, results in asecond order intermodulation in the upstream frequency space at 86,250Hz. This intermodulation is identified as a monitor signal/tone which asdiscussed above is used to monitor and set up the pre-distortion systemof the current invention. The transmission of the second of these tonepairs 412 in the downstream frequency space, results in a third orderintermodulation in the upstream frequency space at the same monitorfrequency/tone of 86,250 Hz.

FIG. 4B is a table showing the selected DMT tones for the downstreamtraining sequence which result in a single monitor tone in the upstreamchannel.

FIG. 5 is a process flow diagram of the setup, configuration andoperational phases of the pre-distortion system of the currentinvention. These processes may be carried out in a single physical modemor in the logical modems discussed above in FIGS. 1-2. Processing beginsat start block 500 from which control is passed to processes 502. Inprocess 502 a leakage model which correlates a level of in band tonepairs on the transmit path with the expected level of the correspondingmonitor tone on the receive path is uploaded. This may be derived usingan electronic simulation package such as Spice or may be empirically orexperimentally derived. Next, control passes to process 504 in which thedevice model for one or more of the transmit path components is alsouploaded. The device model for the power amplifier/line driver would bea mathematical model supplied by the manufacturer or empirically orexperimentally derived. Then control passes to process 506 in which theinitial tone set (Pair) is generated (See FIG. 4B). Next while thefirst/next tone set is being transmitted variations to deviceparameters, e.g. voltage or bias are incremented to a first/next valuein process 508. Then in process 510 the upstream path is monitored forthe out-of-band monitor tone. The level of that tone or the distortionthereof is recorded in a distortion table. Then in decision process 512a determination is made as to whether the device parameter has beenincremented across all possible values during the generation of theselected tone set. If not a new device parameter is selected in process514 and control returns to process 510 for the monitoring and recordingof the relevant parameters of the out-of-band monitor signal generatedthereby. Alternately, after all device parameter variations for aselected tone set have been made control passes to decision process 516.In decision process 516 a determination is made as to whether the lasttone set of the training sequence has been generated (See FIG. 4B). Ifnot control passes to process 518 for the selection of the next toneset, after which control returns to process 508. Alternately, if thelast tone set has been generated during the setup phase then controlpasses to process 520.

In process 520 the analog parameters that min distortion across atraining sequence are determined from the values stored in thedistortion table 510. Control then passes to process 522 in which theselected analog control parameters are locked in to the correspondingone(s) of the transmit path components, e.g. the line driver oramplifier. Then control passes to process 524 which is the first of theprocesses associated with the configuration phase.

In process 524 the training sequence of the plurality of tone sets isagain generated, this time producing a monitor signal/tone at reducedlevels resulting from the selection processes of the above discussedsetup phase. Next in process 526 for each tone set a correspondingestimate of the monitor signal produced thereby is calculated using theleakage model and the device model discussed above. Next in process 528the estimated level of the monitor signal/tone is compared with theactual level of the monitor signal/tone on the receive path and theerror between the two is recorded in the appropriate row of the devicemodel table in process 540. Control then passes to decision process 542in which a determination is made as to whether the training sequence iscomplete. If it is control passes to process 544 for the computation ofthe inverse non-linear channel model from the values stored in thedevice model table. This marks the end of the calibration phase.

Finally, in process 546 the inverse non-linear channel model is appliedduring the operational phase to the data transmitted by the modem. Thispre-distortion linearizes the output of the transmission path improvingtone recovery on the opposing end of the subscriber line, whileminimizing out-of-band interference on the receive path.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

1. A multi-tone communication device with a transmit path and a receivepath configured to couple to a communication medium and with thetransmit path and the receive path including transmit components andreceive components respectively, the communication device comprising: atraining module for transmitting on the transmit path a trainingsequence comprising pairs of tones transmitted on the transmit pathresulting in an intermodulation which generates a single tonecorresponding with a monitor signal on the receive path; a controllerfor controlling variations of at least one control parameter of at leasta selected one of the transmit components during the transmission of thetraining sequence by the training module; a tone detector on the receivepath to detect levels of the monitor signal; and a processor whichutilizes the detected levels of the monitor signal to determine whichamong the variations of the at least one control parameter minimizesleakage between the transmit path and the receive path.
 2. Thecommunication device of claim 1, the controller further for controllingthe locking of the at least one control parameter at the detected levelsdetermined by the processor to minimize leakage during subsequentoperation of the communication device.
 3. The communication device ofclaim 1, wherein the controller controls variations of at least one of,a bias voltage, an input voltage and a temperature of a selectedamplification component within the transmit path of the communicationdevice.
 4. The communication device of claim 1, further comprising onthe receive path: an analog-to-digital converter (ADC) for digitizingthe monitor signal; the tone detector coupled to the ADC for detectingan amplitude of the monitor signal; and a memory for storage by theprocessor of the amplitude of the monitor signal and variations of theat least one control parameter corresponding thereto.
 5. Thecommunication device of claim 1, wherein the communication mediumcomprises one of: a wired medium and a wireless medium.
 6. Thecommunication device of claim 1, wherein the communication devicecomprises one of: a physical modem and a logical modem.
 7. An X-DSLcommunication device with a transmit path and a receive path configuredto couple to a communication medium and with the transmit path and thereceive path including transmit components and receive componentsrespectively, the X-DSL communication device comprising: a trainingmodule for transmitting on the transmit path a training sequencecomprising pairs of tones distributed across a downstream set of X-DSLtones resulting in an intermodulation which generates a single tonecorresponding with a monitor signal within an upstream set of X-DSLtones; a controller for controlling variations of at least one controlparameter of at least a selected one of the transmit components duringthe transmission of the training sequence by the training module; a tonedetector on the receive path to detect levels of the monitor signal; anda processor which utilizes the detected levels of the monitor signal todetermine which among the variations of the at least one controlparameter minimizes leakage between the transmit path and the receivepath.
 8. A communication device with a transmit path and a receive pathconfigured to couple to a communication medium and with the transmitpath and the receive path including transmit components and receivecomponents respectively, the communication device comprising: a trainingmodule for transmitting on the transmit path a training sequence whichgenerates a monitor signal on the receive path; a controller forcontrolling variations of at least one control parameter of at least aselected one of the transmit components during the transmission of thetraining sequence by the training module; a tone detector on the receivepath to detect levels of the monitor signal; a processor which utilizesthe detected levels of the monitor signal to determine which among thevariations of the at least one control parameter minimizes leakagebetween the transmit path and the receive path; and an error detectorfor detecting a difference between an actual level of the monitor signalfrom an ADC and an estimated level of the monitor signal as estimated bythe processor based on a leakage model of the leakage from the transmitpath to the receive path together with an initial device model; theprocessor both updating the initial device model to offset thedifference and subsequently generating an inverse channel model forpredistorting signals on the transmit path to linearize the output ofthe communication device.
 9. The communication device of claim 8,wherein the communication medium comprises one of: a wired medium and awireless medium.
 10. The communication device of claim 8, wherein thecommunication device comprises one of: a physical modem and a logicalmodem.
 11. A method for operating a multi-tone communication device witha transmit path and a receive path configured to couple to acommunication medium and with the transmit path and receive pathincluding transmit components and receive components respectively; themethod comprising: transmitting on the transmit path a training sequencecomprising selected pairs of tones distributed across a downstream setof tones resulting in an intermodulation which generates a single tonecorresponding with a monitor signal on the receive path; varying a levelof at least one control parameter of at least a selected one of thetransmit components during said transmitting step; monitoring during thevarying step the monitor signal on the receive path; determining on thebasis of the monitoring of the monitor signal the level of the at leastone control parameter which minimizes the leakage of the trainingsequence onto the receive path; and utilizing the level of the at leastone control parameter determined in said determining step duringsubsequent transmissions.
 12. The method of claim 11, wherein theselected one of the transmit components includes an amplifier andwherein the at least one control parameter varies in said varying stepincludes at least one of: a bias voltage, an input voltage, and atemperature.
 13. The method of claim 11, wherein the monitoring stepfurther comprises the steps of: digitizing the monitor signal receivedon the receive path; detecting an amplitude of the monitor signal; andstoring the amplitude of the monitor signal detected in the detectingstep for each variation of the level of the at least one controlparameter during said transmitting step the training sequence.
 14. Themethod of claim 11, wherein the communication medium comprises one of: awired medium and a wireless medium.
 15. The method of claim 11, whereinthe communication device comprises one of: a physical modem and alogical modem.
 16. A method for operating an X-DSL communication devicewith a transmit path and a receive path configured to couple to acommunication medium and with the transmit path and the receive pathincluding transmit components and receive components respectively, themethod comprising: transmitting on the transmission path a trainingsequence comprising selected pairs of tones distributed across adownstream set of X-DSL tones an intermodulation of which generates asingle tone corresponding with a monitor signal within an upstream setof X-DSL tones; varying a level of at least one control parameter of atleast a selected one of the transmit components during said transmittingstep; monitoring during the varying step the monitor signal on thereceive path; determining on the basis of the monitoring of the monitorsignal the level of the at least one control parameter which minimizesthe leakage of the training sequence onto the receive path; andutilizing the level of the at least one control parameter determined insaid determining step during subsequent transmissions.
 17. A method foroperating a communication device with a transmit path and a receive pathconfigured to couple to a communication medium and with the transmitpath and the receive path including transmit components and receivecomponents respectively, the method comprising: transmitting on thetransmission path a training sequence which generates a monitor signalon the receive path; varying a level of at least one control parameterof at least a selected one of the transmit components during saidtransmitting step; monitoring during the varying step the monitor signalon the receive path; determining on the basis of the monitoring of themonitor signal the level of the at least one control parameter whichminimizes the leakage of the training sequence onto the receive path;utilizing the level of the at least one control parameter determined insaid determining step during subsequent transmissions; estimating thelevel of the monitor signal throughout the training sequence using adevice model table for the components on the transmit and receive paths;retransmitting the training sequence; monitoring during theretransmitting step an actual level of the monitor signal on the receivepath; determining an error between the level of the monitor signalestimated in the estimating step and the actual level monitored in themonitoring step and updating the device model table accordingly;generating an inverse channel model to linearize an output of thetransmit path utilizing the device model table; and predistortingsignals transmitted in the transmit path utilizing the inverse channelmodel generated in the generating step.
 18. The method of claim 17,wherein the communication medium comprises one of: a wired medium and awireless medium.
 19. The method of claim 17, wherein the communicationdevice comprises one of: a physical modem and a logical modem.
 20. Ameans for configuring a multi-tone communication device with a transmitpath and a receive path configured to couple to a communication mediumand with the transmit path and the receive path including transmitcomponents and receive components respectively, the means comprising:means for transmitting on the transmit path a training sequence whichcomprises pairs of tones distributed across a downstream set of tonesresulting in an intermodulation which generates a single tonecorresponding with a monitor signal within an upstream set of tonesreceived on the receive path; means for varying a level of at least onecontrol parameter of at least a selected one of the transmit componentsduring the training sequence; means for monitoring in the varying meansthe monitor signal on the receive path; means for determining on thebasis of the monitoring of the monitor signal the level of the at leastone control parameter which minimizes the leakage of training sequenceonto the receive path; and means for utilizing the level of the at leastone control parameter determined in said determining means duringsubsequent transmissions.
 21. The means for configuring of claim 20,wherein the pairs of tones are distributed across a downstream set ofX-DSL tones and the single tone corresponding with the monitor signal iswithin an upstream set of X-DSL tones received on the receive path.